1. Field of Invention
This invention relates to methods and devices for improved ionization, collection and focusing of ions generated from chemical and photo-ionization for introduction into the mass spectrometer and other gas-phase ion analyzers and detectors.
2. Description of Prior Art
The generation of ions at or near atmospheric pressure is accomplished by a variety of means; including, electrospray (ES), atmospheric pressure chemical ionization (APCI), atmospheric pressure matrix assisted laser desorption ionization (AP-MALDI), discharge ionization, 63Ni sources, inductively coupled plasma ionization, and photoionization. A general characteristic of these atmospheric or near atmospheric ionization sources is the dispersive nature of the ions once produced. Needle sources such as electrospray and APCI disperse Ions radially from the axis in high electric fields emanating from needle tips. Aerosol techniques disperse ions in the radial flow of gases emanating from tubes and nebulizers. Even desorption techniques such as atmospheric pressure MALDI will disperse ions in a solid angle from a surface. The radial cross-section of many dispersive sources can be as large as 5 or 10 centimeters in diameter.
As a consequence of a wide variety of dispersive processes, efficient sampling of ions from atmospheric pressure sources to small cross-sectional targets or through small cross-sectional apertures and tubes (usually less than 1 mm) into a mass spectrometer becomes quite problematic. This is particularly amplified if the source on ions is removed from the regions directly adjacent to the aperture.
The simplest approach to sampling dispersive atmospheric sources is to position the source on axis with a sampling aperture or tube. The sampling efficiency of simple plate apertures is generally less than 1 ion in 104. Devices developed by Fite (U.S. Pat. No. 4,209,696) used pinhole apertures in plates with electrospray. Devices developed by Laiko and Burlingame (W.O. Pat. No. 99/63576 and U.S. Pat. No. 5,965,884) used aperture plates with atmospheric pressure MALDI. An atmospheric pressure source by Kazuaki et al (Japan Pat. No. 04215329) is also representative of this inefficient approach. This general approach in severely restricted by the need for precise aperture alignment and source positioning, for example, in the case of an APCI source misalignment of the discharge needle can lead to very poor sampling efficiencies.
Recently, a photoionization source has been developed for LC/MS applications by Robb and coworkers (W.O. Pat. No. 01/33605 A2 and U.S. Pat. No. 6,534,765). The use of low field photo-ionization sources has lead to some improvement in sampling efficiency from atmospheric pressure sources, but these sources also suffer from a lower concentration of reagent ions when compared to traditional APCI sources.
A wide variety of source configurations utilize conical skimmer apertures in order to improve collection efficiency over planar devices. This approach to focusing ions from atmospheric sources is limited by the acceptance angle of the electrostatic fields generated at the cone. Generally, source position relative to the cone is also critical to performance, although somewhat better than planar apertures. Conical apertures are the primary inlet geometry for commercial ICP/MS with closely coupled and axially aligned torches. Examples of conical-shaped apertures are prevalent in ES and APCI (U.S. Pat. No. 5,756,994), and ICP (U.S. Pat. No. 4,999,492) inlets. As with planar apertures, source positioning relative to the aperture is also critical to performance; and collection efficiency is quite low.
Another focusing alternative utilizes a plate lens with a large hole in front of an aperture plate or tube for transferring sample into the vacuum system. The aperture plate is generally held at a high potential difference relative to the plate lens. The configuration creates a potential well that penetrates into the source region and has a significant improvement in collection efficiency relative to the plate or cone apertures. But, this configuration has a dear disadvantage in that the potential well resulting from the field penetration is not independent of ion source position, or potential. High voltage needles can diminish this well. Off-axis sources can affect the shape and collection efficiency of the well also. Optimal positions are highly dependent upon both flow (liquid and, concurrent and counter-current gas flows) and voltages. They are reasonable well suited for small volume sources such as nanospray while larger flow sources become less efficient and problematic. Because this geometry is generally preferential over plates and cones, it is seen in most types of atmospheric source designs. We will call this approach the “Plate-Well” design which is reported with apertures by Labowsky et al. (U.S. Pat No. 4,531,056), Covey et al. (U.S. Pat. No. 5,412,209) and Franzen (U.S. Pat. No. 5,747,799). There are also many Plate-Well designs with tubes reported by Fenn et al. (U.S. Pat. No. 4,542,293), Goodley et al. (U.S. Pat. No. 5,559,326), and Whitehouse et al. (U.S. Pat. No. 6,060,705).
Several embodiments of atmospheric pressure sources have incorporated grids in order to control the sampling of gas-phase ions. Jarrell and Tomany (U.S. Pat. No. 5,436,446) utililized a grid that reflected lower mass ions into a collection cone and passed large particles through the grid. This modulated system was intended to allow grounded needles and collection cones or apertures, and float the grid at high alternating potentials. This device had limitations with duty cycle of ion collection in a modulating field (non-continuous sample introduction) and spacial and positioning restrictions relative to the sampling aperture. Andrien et al (U.S. Pat. No. 6,207,954 B1) used grids as counter electrodes for multiple corona discharge sources configured in geometries and at potentials to generated ions of opposite charge and monitor their interactions and reactions. This specialized reaction source was not configured with high field ratios across the grids and was not intended for high transmission and collection, rather for generation of very specific reactant ions. An alternative atmospheric pressure device by Yoshiaki (JP10088798) utilized on-axis hemispherical grids in the second stage of pressure reduction. Although the approach is similar to the present device in concept, it is severely limited by gas discharge that may occur at these low pressures if higher voltages are applied to the electrodes and the fact that most of the ions (>99%) formed at atmospheric pressure are lost at the cone-aperture from atmospheric pressure into the first pumping stage.
Grids are also commonly utilized for sampling ions from atmospheric ion sources utilized in ion mobility spectrometry (IMS). Generally, for IMS analysis ions are pulsed through grids down a drift tube to a detector as shown in Kunz (U.S. Pat. No. 6,239,428B1). Great effort is made to create a planar plug of ions in order to maximize resolution of components in the mobility spectrum. These devices generally are not continuous, nor do they require focusing at extremely high compression ratios.